Multi-phase and shifted phase power distribution systems

ABSTRACT

A method and apparatus for providing a source of alternating current electrical power to a plurality of loads in an office environment, including a plurality of non-linear loads which draw power for only a portion of the alternating current cycle, includes an input source of three-phase electrical power to a primary side of a power transforming device which provides an output at its secondary side which comprises at least six phases and a shared neutral. The plurality of loads are evenly distributed between each phase of the output power source and the shared neutral so as to reduce by current cancellation the current which would otherwise flow in the shared neutral conductor due to the presence of the non-linear loads. Each of the six phases is separated from the other by 120 electrical degrees, and the fourth, fifth and sixth phases are separated from the first, second and third phases, respectively, by 180 electrical degrees. In a preferred embodiment, two sets of six phases are provided, with each of the phases in the first set shifted, relative to respective phases of the second set, so as to reduce variations in the level of instantaneous power drawn from the input source which would otherwise occur due to the presence of the non-linear loads. Benefits of the invention include lower ground to neutral noise, reduced transmission losses, more accurate power measurement by induction watt-hour meters, and fewer instances of neutral current overload.

BACKGROUND AND SUMMARY OF THE INVENTION

The present invention relates generally to power distribution systemsand, more particularly, to power distribution systems for offices andother environments in which power is supplied to a large number ofcomputers or other pulsed, non-linear electrical loads.

Office power distribution systems supply electrical power to a varietyof single phase and three-phase electrical loads. Typical loads have, inthe past, included motors, lighting fixtures, and heating systems. Theseloads are, for the most part, linear in nature. When an alternatingcurrent is applied to a linear load, the current increasesproportionately as the voltage increases and decreases proportionatelyas the voltage decreases. Resistive loads operate with a power factor ofunity (i.e., the current is in phase with the voltage). In inductivecircuits, current lags voltage by some phase angle resulting in circuitswhich operate with a power factor of less than one. In a capacitivecircuit, the current leads the voltage. However, in all of thesecircuits, current is always proportional to the voltage and, when asinusoidal voltage is applied to the load, the resulting current is alsosinusoidal.

Until recently, almost all loads found in a typical office environmentwere linear loads. However, computers, variable speed motor drives, andother so-called "electronic" loads now comprise a significant andgrowing portion of the electrical load present in offices. Theseelectronic loads are, for the most part, non-linear in nature. Theseloads have become a significant factor in many office power distributionsystems, and their presence has lead to a number of problems and officepower system malfunctions.

A non-linear load is one in which the load current is not proportionalto the instantaneous voltage and, in many cases, is not continuous. Itmay, for example, be switched on for only part of a 360 electricaldegree alternating current cycle.

The presence of non-linear loads on a power system can cause numerousproblems. Typical office power distribution systems operate asthree-phase 208/120 volt systems with a shared neutral conductor servingas a return path for currents from each of the three phases. Linearloads which are balanced among the three phases produce currents whichtypically cancel in the shared neutral conductor resulting in relativelylow net current flow in the neutral. Pulsed currents produced bynon-linear loads do not cancel in the neutral conductor because theytypically do not occur simultaneously. These currents tend to add on theneutral even when the three phases of the system are carefully balanced.The resulting high current flows in the neutral conductor can lead tosevere overheating or burnout of neutral conductors, and increased noiselevels on the neutral. Pulsed, non-linear currents further causerelatively large variations in the instantaneous power demanded from thegenerator. These variations can cause problems and inefficiencies on thegenerator and distribution side of the transforming device. Moreover,pulsed, non-linear currents may cause typical induction watt-hour metersto show large calibration errors.

An object of the present invention is to provide a power distributionsystem for an office environment in which the adverse affects of pulsed,non-linear loads are reduced.

This object is achieved in a power distribution system in whichthree-phase electrical power is supplied to a primary side of a powertransforming device, and in which at least six phases and a sharedneutral conductor are provided at the secondary side of the transformingdevice. A plurality of electrical loads, including non-linear loads, aredistributed between each of the six phases and the shared neutral so asto reduce by current cancellation the current which would otherwise flowin the shared neutral conductor due to the presence of the non-linearloads. Each of the first, second and third of the six phases provided atthe output of the transforming device are preferably separated from eachother by 120 electrical degrees. The fourth, fifth and sixth of thesephases are also preferably separated from each other by 120 electricaldegrees, and are from the first, second and third phases, respectively,by 180 electrical degrees. In a particularly preferred embodiment of theinvention, at least 12 phases are produced at the secondary side of thepower transforming device. Each of these 12 phases is preferablyseparated from the other phases by 30 electrical degrees In thisembodiment, the 12 phases may be viewed as two sets of six phases, witheach of the phases in a first of the two sets shifted relative torespective phases of the second set, so as to reduce variations in thelevel of instantaneous power drawn from the input source which wouldotherwise occur due to the presence of the non-linear loads. In thispreferred embodiment, the six phases in the first set are preferablyshifted by 30 electrical degrees relative to respective ones of the sixphases in the second set. This may be advantageously accomplished byshifting each set of six phases by 15 electrical degrees in oppositedirections relative to the phase angles of the incoming power source.

Other objects, advantages and novel features of the present inventionwill become apparent from the following detailed description of theinvention when considered in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a switched power supply circuit of the type commonly usedin devices such as personal computers.

FIGS. 2(a) and 2(b) show the waveforms of the input line voltage andline current associated with power supply circuit 10 of FIG. 1.

FIG. 3 shows a schematic wiring diagram of a prior art office powerdistribution system.

FIG. 4a graphically illustrates the line voltages present on each ofphases L1, L2 and L3 in the circuits of FIG. 3.

FIG. 4b graphically illustrates the current waveform for each phase ofthe circuits shown in FIG. 3 when a power supply circuit for the typeshown in FIG. 1 is connected between each phase and the shared neutral.

FIG. 4c graphically illustrates the magnitude of the current flowing onthe shared neutral conductor as a result of the currents illustrated inFIG. 4b.

FIG. 5 shows a schematic wiring diagram of a power distribution systemof the present invention.

FIG. 6 is a vector phase diagram which illustrates the phase separationexisting between phases L1-L6 of FIG. 5.

FIG. 7 graphically illustrates the current waveform for each of phasesL1-L6 of FIG. 5 when a power supply circuit of the type illustrated inFIG. 1 is connected between each phase and a shared neutral conductor.

FIG. 8 graphically illustrates variations in the level of power drawn bythe loads connected between phases L1-L6 of FIG. 5.

FIG. 9 graphically illustrates the variations in the level of powerdrawn by the loads connected between phases L1-L6 and the shared neutralof FIG. 5, and by an identical set of loads identically connectedbetween phases L7-L12 of FIG. 5.

FIG. 10 shows a summation of the waveforms illustrated in FIG. 8.

FIG. 11 schematically illustrates a transforming device constructed inaccordance with the principles of the present invention.

FIG. 12 shows a schematic wiring diagram of an alternative embodiment ofa power distribution system constructed in accordance with theprinciples of the present invention.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a switched power supply circuit 10 of the type commonlyused in personal computers. In power supply circuit 10 of FIG. 1, linevoltage is rectified by a bridge rectifier 12 at the input of circuit10. The resulting DC power charges capacitor C. A chopping circuit 14 isused to convert the resulting DC power back to AC power for subsequenttransformation and regulation as required by the particular deviceincorporating the power supply.

FIGS. 2(a) and 2(b) show the waveforms of the input line voltage andline current associated with power supply circuit 10 of FIG. 1. Sincethe diodes in bridge circuit 12 conduct only when the forward biasingvoltage exceeds the voltage across capacitor C, line current flows intopower supply circuit 10 in accordance with the waveform shown in FIG.2(b). As shown, the line current drawn by power supply circuit 10consists of a series of positive and negative peaks which are alignedwith the positive and negative peaks of the line voltage, and which areseparated by relatively long periods during which no line current flows.The "dwell" or conduction angle of each peak is typically 40-50electrical degrees, but will vary in accordance with the demand forpower at the output of power supply circuit 10.

FIG. 3 shows a schematic wiring diagram of a power distribution systemfor office power systems commonly used in the prior art. In thearrangement shown in FIG. 3, three-phase power is supplied from utilitylines 20 at a relatively high voltage to the primary of a transformingdevice 22. The secondary of transforming device 22 provides three-phasepower, typically at 480 volts, via conductors 24 to a service entranceor panel 26 of a customer. In this instance, service panel 26 may belocated in an office building 28, schematically represented by dashedlines in FIG. 3. Connected to the output side of panel 26 are aplurality of distribution circuits represented generally by circuits 29and 30. Circuits 29 and 30 typically include three-phase transformingdevices 32 and 34, respectively. Electrical power is provided to theprimary sides of transforming devices 32 and 34 from panel 26 at 480volts (line to line) and is stepped down to voltage levels of 208 volts(line to line) and 120 volts (line to neutral). The secondary or outputsides of transforming devices 32 and 34 are connected to a variety ofloads, including lighting loads, computers and convenience outlets.Loads are typically connected between one of the three line conductorsL1, L2 and L3, and a shared neutral conductor N. A separate groundconductor is also provided. Voltages on lines L1, L2 and L3 are 120electrical degrees out of phase. When resistive loads are connectedbetween each phase conductor and the shared neutral in a balancedmanner, no current flows in the shared neutral due to currentcancellation effects resulting from the relative phase relationshipsexisting between the voltages on line conductors L1, L2 and L3.

FIG. 4(a) graphically illustrates the line voltages present on each ofphases L1, L2 and L3 in the circuits of FIG. 3. As illustrated, eachphase is separated from, or shifted relative to, the other two phases by120 electrical degrees. FIG. 4(b) graphically illustrates the currentwaveform for each phase when a power supply circuit of the typeillustrated in FIG. 1 is connected between each phase conductor and theshared neutral conductor N. As discussed in connection with FIGS. 1 and2 above, line current flows in each phase for only a portion of eachhalf cycle due to the design of the power supply circuit.

FIG. 4(c) graphically illustrates the magnitude of the current flowingon the shared neutral conductor N as a result of the currentsillustrated in FIG. 4(b). As is apparent from FIG. 4(c), all of thecurrent flow is present in the shared neutral conductor, notwithstandingthe fact that equal loads are connected between each phase and theneutral (i.e., the loads are balanced). Due to the "pulsed" nature ofthe current flow occurring in each phase, current cancellation effectswhich might otherwise reduce or eliminate current flowing in the sharedneutral conductor do not reduce or eliminate current in the neutral inthis instance. As long as the conduction angles of the pulsesillustrated in FIG. 4(b) are 60° or less, current cancellation will notoccur in the neutral conductor since there is no "overlapping" ofcurrents from the individual phases.

FIG. 5 shows an electrical power distribution system in which theproblem of excessive neutral conductor currents of the type illustratedin FIG. 4(c) is addressed. Elements 20-28 of FIG. 5 are essentiallyidentical to corresponding elements of FIG. 3 and, thus, have beennumbered accordingly. Circuits 36 and 38 differ, however, from circuits29 and 30. Specifically, circuits 36 and 38 include transforming devices40 and 42 which transform the three-phase, 480 volt input power to asix-phase, 208 volt (line-to-line) output to provide six phases (L1, L2,L3, L4, L5 and L6 in circuit 36 and L7, L8, L9, L10, L11 and L12 incircuit 38), each of which is separated by 60 electrical degrees fromthe others. FIG. 6 is a vector phase diagram which illustrates the phaseseparation existing between phases L1-L6. As illustrated in FIG. 6,phases L1, L2 and L3 are separated by 180 electrical degrees from phasesL4, L5 and L6, respectively.

FIG. 7 illustrates the current flowing in each of phases L1-L6 when apower supply circuit of the type illustrated in FIG. 1 is connectedbetween each of these phase conductors and the shared neutral conductorN. As illustrated in FIG. 7, each of the current peaks caused by currentflow in phases L1, L2 and L3 is offset by an equal and opposite currentflowing in phases L4, L5 and L6. Thus, the net current flow in theshared neutral conductor N as a result of the loads illustrated in FIG.7 is zero. In other words, "pulsed" currents of the type illustrated inFIG. 2(b) flowing in the shared neutral conductor as a result of theloads connected between phases L1, L2 and L3 and the shared neutral areoffset or cancelled by equal and opposite currents flowing in the sharedneutral due to similar loads connected between phases L4, L5 and L6 andthe shared neutral. To the extent the loads on phases separated by 180electrical degrees are identical, all currents, including thefundamental and all harmonics, cancel. The currents illustrated in FIG.7 are, of course, idealized. In practical applications, "perfect"balance between opposing phases will rarely be achieved and is notnecessary to provide the benefit of substantial reductions of currentwhich might otherwise flow in the neutral conductor due to the presenceof pulsed, non-linear loads in the system. References to "balanced"loads or "balancing" of loads in this application are not to be taken asrequiring that precisely the same number of loads be connected betweeneach phase and the shared neutral conductor.

The six-phase arrangement described above may be used to effectivelyreduce or eliminate excess neutral currents flowing in the neutralconductor on the load side of transforming devices 40 and 42 of circuits36 and 38, respectively. Illustrated in FIG. 8, however, is a separateproblem which still exists on the generator side of transforming devices40 and 42. FIG. 8 shows a graphical representation of the instantaneouspower demanded by circuits 36 and 38 operating with the loadsillustrated in FIG. 7. Unlike the neutral currents on the load side ofdevices 40 and 42, the instantaneous power demanded by each loadconnected between phases L1-L6 and the shared neutral N does not cancel,but is additive to produce the "pulsed" power demand illustrated in FIG.8. This type of power demand is more difficult for an electric utilityto satisfy than is an essentially constant, steady demand. Indeed, insome locations, utility customers connecting such loads to the utilitysystem will be penalized in the form of higher rates or otherassessments.

The condition illustrated in FIG. 8 can be addressed, and substantiallyimproved, by shifting the respective phases of circuits 36 and 38relative to one another. In other words, phases L7-L12 of circuit 38 canbe shifted relative to the respective phases L1-L6 of circuit 36 tosmooth the overall demand for power, as viewed from the generator sideof transforming devices 40 and 42. FIG. 9 illustrates the power demandedby circuits 36 and 38, respectively, after each of the phases L7-L12 ofcircuit 38 are shifted by 30 electrical degrees relative to thecorresponding phases L1-L6 of circuit 36. FIG. 10 illustrates the sum ofthe two waveforms shown in FIG. 9, and thus illustrates the power demandas seen by the utility system on the generator side of transformingdevices 40 and 42. As is readily apparent, the power demand illustratedin FIG. 10 is much more constant and steady than that illustrated inFIG. 8 (the difference between the peak value and the average value ofthe waveform of FIG. 10 is approximately 8%). This appears much morelike a resistive load to the generator. In addition to easing otherproblems on the generator side of the transformer, this smoothing of thepower demand tends to correct the large calibration shifts which occurin inductive watt-hour meters due to the presence of the pulsed,non-linear load currents.

As noted, FIG. 9 illustrates the power demanded by circuits 36 and 38,respectively, after each of the phases of circuit 38 are shifted by 30electrical degrees relative to the corresponding phases of circuit 36.There are several ways in which this relative phase shift can beaccomplished. However, it may be advantageous to achieve this relativeseparation by shifting phases L1-L6 of circuit 36 by 15 electricaldegrees in one direction (relative to, for instance, the incomingphases), and shifting phases L7-L12 of circuit 38 by 15 degrees in theopposite direction. This arrangement may result in additional"smoothing" of the instantaneous power demanded from the generator dueto the likely presence of other loads which are "in-phase" with theincoming power source.

Although the arrangement in FIG. 5 utilizes two transforming devices (40and 42), the respective outputs of which are phase shifted to reduce therelative magnitudes of the power "pulses" on the generator side of thetransformer, other arrangements may be used to accomplish this result.For example, a single transforming device having a twelve-phase outputmay also be used. Further, "smoothing" of the instantaneous powerdemanded from the generator can be accomplished in a system having fewerphases, and in which neutral current cancellation on the load side doesnot occur. For example, a system having a three-phase input and a threephase output in which each of the output phases is shifted, for example,by 30 electrical degrees relative to each of the respective input phaseswill smooth the power demanded from the generator by pulsed, non-linearloads. This arrangement will provide benefits to the utility company (orother power provider), even in the absence of the current cancellationbenefits on the load side discussed above.

FIG. 11 schematically illustrates a transforming device 50 which can beused in accordance with the present invention. Device 50 has adelta-connected primary which provides power to three primary windings52, 54 and 56. The respective secondaries associated with each of theprimary windings comprise main secondary windings 58, 60 and 62, and aplurality of smaller windings labeled T1, T2 and T3, respectively. Thesesmaller windings are connected in series with one or the other of eachside of secondary windings 58, 60 and 62, as illustrated, to provide a12-phase output in which, for example, phases L1-L6 are separated fromeach other by 60 electrical degrees, phases L7-L12 are separated fromeach other by 60 electrical degrees and phases L1-L6 are each shifted 15electrical degrees in one direction (relative to the input power source)and phases L7-L12 are each shifted 15 electrical degrees in the otherdirection.

Using a device such as that illustrated in FIG. 11, loads can bedistributed between phases L1-L6 and L7-L12 so as to effectivelyeliminate, or reduce, current on the shared neutral conductor N on theload side of the transformer. Phases L1-L6 can further be shifted,relative to respective phases L7-L12, to reduce the instantaneousmagnitude of the power demands on the generator side of the transformer.In "ideal" circumstances, loads will be evenly distributed between eachof the phases and neutral to achieve maximum reduction of current in theneutral conductor, and the two groups of six-phases will be uniformlyshifted to smooth the instantaneous power demanded from the generator tothe greatest degree. However, under more realistic conditions, loaddistributions which are not precisely even, and varying degrees ofrelative phase shifting, may be most effective in mitigating theproblems discussed above. The ability to "tune" the system byperiodically re-distributing loads and adjusting relative phase shiftsmay be desirable and justifiable in particular circumstances. In largeinstallations, additional six-phase and/or twelve-phase circuits,utilizing varying degrees of phase shifting, may further reduce thenegative effects caused by large concentrations of loads such as thatshown in FIG. 1.

FIG. 12 shows a schematic wiring diagram of an alternative embodiment ofthe present invention which may be particularly advantageous in smallerpower systems and/or in retrofitting existing installations. The systemillustrated in FIG. 12 is similar in many respects to the prior artsystem illustrated in FIG. 3 and like reference numerals are used toindicate like elements, accordingly. However, in the circuit of FIG. 12,an additional element, in the form of transforming device 70 has beenadded. The primary side of transforming device 70 is connected to thethree-phase output of device 32. The three-phase secondary output ofdevice 70 (i.e., phases L4, L5 and L6) may be shifted by 180 electricaldegrees, relative to phases L1, L2 and L3, respectively, to reduce oreliminate current flow in the shared neutral conductor resulting fromconnection of a plurality of pulsed, non-linear loads between therespective phases and the neutral. Alternatively, each of phases L4, L5and L6 may be shifted by 30 electrical degrees, for instance, to smooththe demand for power on the generator side of device 32. Device 32 mayalso be wound to shift phases L1, L2 and L3 relative to the phases ofthe input power source by, for instance, 15 electrical degrees in thedirection opposite the 30 degree shift effected by device 70.

From the preceding description of the preferred embodiments, it isevident that the objects of the invention are attained. Although theinvention has been described and illustrated in detail, it is to beclearly understood that the same is intended by way of illustration andexample only and is not to be taken by way of limitation. The spirit andscope of the invention are to be limited only by the terms of theappended claims.

What is claimed is:
 1. A method of providing a source of alternatingcurrent electrical power to a plurality of loads, including a pluralityof non-linear loads which draw power from the source for only a portionof the alternating current cycle, comprising the steps of:providing aninput source of three-phase electrical power to a primary side of apower transforming device; transforming the input power source toprovide an output power source at a secondary side of the powertransforming device, said output power source comprising at least sixphases and a shared neutral; and distributing the loads connectedbetween each phase of the output power source and the shared neutral soas to reduce by current cancellation the current which would otherwiseflow in the shared neutral conductor due to the presence of thenon-linear loads.
 2. A method according to claim 1, wherein a first,second and third of said six phases are separated from each other byapproximately 120 electrical degrees, and wherein a fourth, fifth andsixth of said phases are separated from each other by approximately 120electrical degrees and are separated from the first, second and thirdphases, respectively, by approximately 180 electrical degrees.
 3. Amethod according to claim 1, wherein said output power source comprisesat least twelve phases.
 4. A method according to claim 3, wherein eachof said twelve phases is separated from the other phases byapproximately 30 electrical degrees.
 5. A method according to claim 1,wherein said transforming step includes providing at least two sets ofsix phases, and wherein each of the phases in a first of said two setsof six phases are shifted relative to respective phases of the secondset, so as to reduce variations in the level of instantaneous powerdrawn from the input source which would otherwise occur due to thepresence of the non-linear loads.
 6. A method according to claim 5,wherein said six phases in the first set are shifted by approximately 30electrical degrees relative to respective ones of the six phases in thesecond set.
 7. A method according to claim 5, wherein each of the twosets of six-phases is shifted in opposite directions relative to theinput power source.
 8. A method according to claim 7, wherein each ofthe two sets of six-phases is shifted by approximately 15 electricaldegrees relative to the input power source.
 9. A method of providing asource of alternating current electrical power to a plurality of loads,including a plurality of non-linear loads which draw power from thesource for only a portion of the alternating current cycle, comprisingthe steps of:providing an input source of three-phase electrical powerto a primary side of a power transforming device; transforming the inputpower source to provide an output power source at a secondary side ofthe power transforming device, said output power source comprising atleast two sets of six phases and a shared neutral, and wherein each ofthe phases in a first of said two sets of six phases are shifted,relative to respective phases of the second set of the six phases so asto reduce variations in the level of instantaneous power drawn from theinput source which would otherwise occur due to the presence of thenon-linear loads; and balancing the distribution of loads connectedbetween each phase of the output power source and the shared neutral soas to reduce by current cancellation the current which would otherwiseflow in the shared neutral conductor due to the presence of thenon-linear loads.
 10. A power distribution system for supplyingalternating current electrical power to a plurality of loads, includinga plurality of non-linear loads which draw power from the system foronly a portion of the alternating current cycle, comprising:means forproviding an input source of three-phase electrical power to a primaryside of a power transforming device; and means for transforming theinput power source to provide an output power source at a secondary sideof the power transforming device, said output power source comprising atleast six phases and a shared neutral; wherein the plurality of loadsconnected to the system are distributed between each phase of the outputpower source and the shared neutral so as to reduce by currentcancellation the current which would otherwise flow in the sharedneutral conductor due to the presence of the non-linear loads.
 11. Apower distribution system according to claim 10, wherein a first, secondand third of said six phases are separated from each other byapproximately 120 electrical degrees, and wherein a fourth, fifth andsixth of said phases are separated from each other by approximately 120electrical degrees and are separated from the first, second and thirdphases, respectively, by approximately 180 electrical degrees.
 12. Apower distribution system according to claim 10, wherein said outputpower source comprises at least twelve phases.
 13. A power distributionsystem according to claim 12, wherein each of said twelve phases isseparated from the other phases by approximately 30 electrical degrees.14. A power distribution system according to claim 10, wherein saidtransforming means includes means for providing at least two sets of sixphases, and wherein each of the phases in a first of said two sets ofsix phases are shifted, relative to respective phases of the second set,so as to reduce variations in the level of instantaneous power drawnfrom the input source which would otherwise occur due to the presence ofthe non-linear loads.
 15. A power distribution system according to claim14, wherein said six phases in the first set are shifted byapproximately 30 electrical degrees relative to respective ones of thesix phases in the second set.
 16. A power distribution system accordingto claim 14, wherein each of the two sets of six-phases is shifted inopposite directions relative to the input power source.
 17. A powerdistribution system according to claim 16, wherein each of the two setsof six-phases is shifted by approximately 15 electrical degrees relativeto the input power source.
 18. A power distribution system according toclaim 10, wherein said power transforming device comprises at leastthree primary windings, at least three main secondary windingsassociated, respectively, with the primary windings, and a plurality ofsmaller secondary windings.
 19. A power distribution system according toclaim 18, wherein selected ones of said smaller secondary windings areconnected in series with selected ones of said main secondary windingsto produce the at least six phases of the output power source.
 20. Apower distribution system for supplying alternating current electricalpower to a plurality of loads, including a plurality of non-linear loadswhich draw power from the system for only a portion of the alternatingcurrent cycle, comprising:means for providing an input source ofthree-phase electrical power to a primary side of a power transformingdevice; and means for transforming the input power source to provide anoutput power source at a secondary side of the power transformingdevice, said output power source comprising at least two sets of sixphases and a shared neutral, and wherein each of the phases in a firstof said two sets of six phases are shifted, relative to respectivephases of the second set of the six phases so as to reduce variations inthe level of instantaneous power drawn from the input source which wouldotherwise occur due to the presence of the non-linear loads; and whereinthe plurality of loads are connected between each phase of the outputpower source and the shared neutral so as to reduce by currentcancellation the current which would otherwise flow in the sharedneutral conductor due to the presence of the non-linear loads.
 21. Apower distribution system according to claim 20, wherein said powertransforming device comprises at least three primary windings, at leastthree main secondary windings associated, respectively, with the primarywindings, and a plurality of smaller secondary windings.
 22. A powerdistribution system according to claim 21, wherein selected ones of saidsmaller secondary windings are connected in series with selected ones ofsaid main secondary windings to produce the 12 phases of the outputpower source.
 23. A method of providing a source of alternating currentelectrical power to a plurality of loads, including a plurality ofnon-linear loads which draw power from the source for only a portion ofthe alternating current cycle, comprising the steps of:providing aninput source of three-phase electrical power to a primary side of apower transforming device; transforming the input power source toprovide an output power source at a secondary side of the powertransforming device, said output power source comprising at least onephase; and shifting the output phase relative to the input phase so asto reduce the level of instantaneous current drawn from any one of theinput phases due to the presence of the non-linear loads.